In modern dentistry, 3-D shapes of teeth and the positions of roots are very important in computer assisted procedures, such as surgical pre-operative planning and navigation, mechanized dental implant, post-operative assessments and cosmetic surgeries.
Existing literature for teeth segmentation is mostly limited to the data format of conventional and/or panoramic X-ray imaging. In the last decade, CT has become the most frequently used imaging modality to provide clinical datasets for dental preoperative and intra-operative planning.
A general interactive segmentation framework includes a friendly user interface which enables users to impose certain hard constraints, anywhere within the image domain, to mark certain pixels to be absolutely part of the image foreground and background, respectively. Although the total number of marked pixels is very limited, sometimes no more than several hundred, they are the interactive clues which are able to be utilized by a graph cuts segmentation algorithm. By presenting the instant results to the user, second-round interaction is to be given again if there is a need to make modifications. This interaction procedure repeats until the user feels satisfied. FIG. 1 illustrates this procedure.
Regarding interaction with users, people are seeking for loose inputs to mark foreground and background. Loosely positioned marking lines, like the strokes given by brush are more preferable than precise boundary definition. An exemplary case is shown in FIG. 2, where red (200) and blue (202) strokes are indicating foreground and background, respectively.
3-D CT volumetric image suffers from significant partial volume effect (PVE), a frequently-occurred phenomena due to the limited spatial resolution of the CT imaging device and the complex shape of tissue interfaces. As a consequence, the CT intensities of those voxels near tissue interfaces are actually mixtures of more than one tissue type which impose extra challenges for developing computerized CT segmentation algorithm.
Besides PVE, the 3-D teeth CT dataset has its own characteristics. First of all, teeth are bony tissue; however, not the only bony tissue present in the dental CT region. Other surrounding bony tissues include upper and lower jaws. Secondly, the CT intensities of teeth area, from top crown to bottom root, vary a lot. Finally, due to teeth's dense spatial arrangement, one tooth is connected to its neighboring teeth in the top crown part.